Paperboard cartons are commonly used for distributing milk and juice. Typically, paperboard cartons are formed from flat blanks that have a central layer of paperboard and outer layers of polyethylene. A gas barrier layer may be included in the laminate. A common form of these cartons is referred to as a gable-top carton. This type of carton has four side walls and a flat bottom. The top of the carton is folded into a closing arrangement which resembles a roof. Along the ridge of the roof, a sealing fin projects upwardly. The laminate that forms the sealing fin is heated to soften the polyethylene. When the laminate is then pressed together along the sealing fin and cooled, so that the polyethylene layers form a secure bond.
In order to open the sealed carton, the sides of the roof are pulled apart to separate each half of the sealing fin between the end of the roof and the center of the ridge of the roof. The other half of the sealing fin remains sealed. The second step in the opening process requires unsealing the inside layers of the sealing fin, and this is done by swinging the sides of the roof further away from each other past the middle of the roof ridge, and then to urge the two sides toward each other with sufficient force to break the seal between the inside layers of the seal fin, which then swing outwardly to form a pouring spout.
Although the gable-top carton is used extensively, the opening arrangement has been criticized by consumers for the difficulty in opening and forming the pouring spout. Often, in conventional cartons it is necessary to actually pull the spout away from the center of the sealing fin with sufficient force to break the seal between the layers of the seal fin. When the seal fin is torn apart, all of the polyethylene may adhere to one side of the seal and, as a result, the rough surface of the paper is exposed. This rough paper surface is not only visually objectionable, it may be the source of bacteria if the carton remains open for a long time after its first use.
Various attempts have been made to improve the opening arrangement. One suggestion for improving the opening arrangement is disclosed in U.S. Pat. No. 4,712,727 which involves applying an adhesive-coated plastic strip on the inside of the carton blank overlapping the sealing fin. The plastic tape reduces the adhesion between the tape and the opposite polyethylene coating, thereby reducing the force required to open the sealing fin. The tape also strengthens the sealing fin so that in the second opening step in which the spout is formed, the sides of the sealing fin are able to withstand any tendency to buckle. This proposed arrangement does not perform satisfactorily, however, in carton-filling and sealing machines that are currently in use. The tape tends to move during the sealing operation, which prevents a hermetic seal and in some cases causes leakage of the contents of the carton to occur. Another problem with utilizing the tape is that in the sealing operation, the tape tends to wrinkle or become uneven. All of these problems lead to an ineffective seal, which permits O.sub.2 permeation or product penetration. For a practical commercial use, the cartons must remain sealed until opened by the customer. For example, if even a minute channel through the sealing fin results from the use of the tape, air may enter the carton and prematurely spoil the contents. Also, in transporting and handling the carton, the contents may leak, particularly along the crease lines at the center of the sealing fin. As a practical matter, the integrity of the seal is essential to providing a commercially-viable carton opening arrangement.
There are various techniques for applying the tape to the surface of the carton blank. Typically, the tape has a coating of adhesive on one side and is bonded to the surface of the carton blank by the adhesive. The use of an adhesive at the location of the pouring spout obviously adds to the thickness of the sealing fin, and requires modification of conventional carton sealing equipment to accommodate the presence of the tape. Another problem is that it is intended that the adhesive at least partially extrude outward from the tape during the sealing operation. Due to the plastic nature of the adhesive, it does not provide a stable support for the tape, and this leads to displacement of the tape, wrinkling and distortion during the sealing operation.